It is known that most airplanes, more specifically airliners, are provided with a collision avoidance system, in particular of the TCAS type (for <<Traffic alert and Collision Avoidance System>>) allowing to ensure the safety of the air traffic while preventing the collision risks in flight.
Such a TCAS collision avoidance system arranged on board an airplane operates independently from the ground air control. It allows to monitor the trajectories of aircrafts in the vicinity of the airplane being considered and to represent the respective positions thereof on a viewing screen. For detecting possible aircrafts in the air space surrounding the airplane, the TCAS collision avoidance system emits interrogation signals in all directions. When an aircraft circulating in such an air space receives such an interrogation signal, it emits in turn a response signal towards the airplane, so as to notify it of its presence.
From information contained in the response signal transmitted by the aircraft, the TCAS collision avoidance system of the airplane is able to determine the value of the parameters relating to this detected aircraft, such as the distance between it and the airplane, the altitude thereof, the angular positioning thereof with respect to said airplane (i.e. the angle defined between the longitudinal axis of the airplane and the straight line crossing the airplane and the aircraft), etc.
However, the accuracy of the angular positioning of the aircraft with respect to the airplane, being determined by the collision avoidance system, is low and frequently has an uncertainty margin of 15°. Such an inaccuracy makes the avoidance maneuver of the aircraft by the airplane risky in a horizontal plane, because of a high uncertainty about the exact position of the aircraft with respect to the airplane. Thus, all the avoidance steps are currently performed in a vertical plane (the airplane climbs or comes down for avoiding an aircraft).
In addition, the altitude positioning of the detected aircraft is obtained from altitude information contained in the reply signal sent by the aircraft. However, when such altitude information is wrong, the obtained altitude positioning proves to be erroneous. The airplane is then not properly positioned in the vertical plane by the collision avoidance system, which could compromise the avoidance maneuver of the latter.